1. Field of the Invention
The present invention relates generally to the field of electric machines and more particularly to an electric machine having laminated or non-laminated cooling rings adjacent end-turns of the stator core windings for transferring heat from the windings to the housing.
2. Background Art
The cooling of stator windings of electric machines is critical for providing satisfactory motor performance, reliability and durability. This issue becomes even more pressing when the motor is required to have a high power density (kW/m3) and low specific weight. An integrated starter/alternator is a good example where the electric motor is required to have high power density, low specific weight, and good cooling capability because of packaging constraints, fuel economy requirements, and the fact that the motor is in the engine compartment Thermally conductive epoxy potting materials have been applied on the end-turn windings to improve the cooling of an electric motor. FIG. 1 illustrates a motor 10 having such a feature. As shown, the motor 10 consists of a rotor 12 rotatably positioned within a stator core 14. The stator core 14 has first and second ends 16, 18 with end-turns 20, 22 of the windings extending from the ends 16, 18, respectively. A housing 24 having end plates 26, 28 encloses the stator core, windings and rotator. A thermally conductive potting material 30 is positioned over each of the end-turns 20, 22 for carrying heat directly from the end-turns 20, 22 to the housing 24 for heat dissipation.
This potting technology provides limited value because the potting material has a fairly high thermal resistance, which limits the heat dissipation capacity. Additionally, the cooling potential of the potting material cannot be fully utilized because of gaps which are developed between the potting material and the winding, and between the potting material and the housing as a result of thermal cycling of the motor. The reason for the gap development is that the potting material, the winding and the housing have different thermal expansion coefficients. The gaps are filled with air, which provides minimal thermal conductivity.
Accordingly, it is desirable to provide an improved method and apparatus for cooling an electric machine.
The present invention improves upon the above-described cooling method by providing two laminated aluminum ring assemblies at the opposing ends of the stator core which are embedded within the potting material. The laminated aluminum rings substantially enhance heat dissipation, while the laminations reduce eddy current buildup within the rings.
In a preferred embodiment, each lamination of the rings has a plurality of radially extending slots which provide air gaps to minimize eddy currents in a plane perpendicular to the central axis of the motor.
More specifically, the present invention provides an electric machine including a stator core having first and second ends and having windings therein with end-turns of the windings protruding from the first and second ends of the stator core. A rotor is rotatably positioned within the stator core. First and second laminated aluminum ring assemblies are positioned against the first and second ends, respectively, of the stator core in contact with the housing. Thermally conductive potting material is positioned between the end-turns and the respective first and second ring assembly at the first and second ends of the stator core, thereby creating heat dissipation paths from the end-turns, through the potting material and the ring assemblies to the housing.
An alternate embodiment of the present invention takes the form of an electric machine that includes, for example, a housing, a stator core disposed within the housing, and a rotor disposed rotatably within the stator core. The stator core has at least one end-turn that extends beyond an axial end of the stator core and is potted with a potting material. A thermal conductor ring, which can be metallic or a non-metallic and that provides thermal conductivity greater than the potting material, is disposed between the potted stator core end-turn and the housing for conducting heat from the stator core end-turn to the housing. Suitable materials for the thermal conductor ring include, for example, aluminum or aluminum alloy. In addition, the housing can have a coolant fluid circulated therein.
This thermal conductor ring that is disposed between the potted stator core end-turn and the housing has an outer face disposed against an inner surface of the housing that is pressed fit against the inner surface of the housing. Exemplarily, this may be through the utilization of thermal expansion and contraction to shrink fit the housing against the outer face of the thermal conductor ring. An example of a suitable range of thermal conductivity for the thermal conductor is at least 90 BTU/hr ft degree F at 300 degrees Kelvin. The stator core end-turn typically has a highly irregular surface so that good contact directly between the thermal conductor ring and the end-turn is not possible. Thus, a space is defined between the end-turn and the thermal conductor that is filled with the potting material. Typically, the potting material is a flexible, elastomeric and/or viscoelastic material that is arranged and utilized to minimize gap formation caused, for example, by thermal cycling of the engine components. Accordingly, one purpose of the invention is to provide an improved method and apparatus for cooling an electric machine wherein laminated aluminum ring assemblies are provided at opposing ends of the stator core for dissipating heat through a potting material from the end-turns of the windings.
It is a further feature and advantage of an alternate embodiment of the present invention to provide an electric machine having cooling rings, such as non-laminated aluminum cooling rings, for cooling the stator core windings of the electric machine.
The features and advantages of the present invention will be readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.